This invention relates to coating techniques utilizing electrostatic fluidized beds and, more particularly, to the penetration of coating powders into holes, slots or other cavities in the substrate and the rate at which the powdered layer is deposited.
In the last several decades, a number of solventless painting processes have been developed in which finely divided, heat-fusible materials are deposited on a substrate and are then fused into continuous functional or decorative film. Representative of these processes are the fluidized bed, the electrostatic spray, and the electrostatic fluidized bed (ESFB).
The present invention relates specifically to electrostatic fluidized beds which, as the name implies, combines certain features of the fluidized bed and the electrostatic deposition processes. More specifically, in the ESFB process, air is introduced into a plenum chamber below the porous distribution plate of a fluidized bed where it passes through a charging medium and is ionized by a corona discharge. The ionized air then passes through the porous plate to fluidize a bed of finely divided coating powder. The ionized charge in the fluidizing gas is transferred to the fluidized particles which, because they all bear a similar charge, form a cloud of charged particles. When a grounded substrate is brought into close proximity with this charged cloud, the coating powder is attracted to and deposited upon the grounded substrate. Subsequent to this deposition, the substrate is heated in a convection oven or by other means to fuse the particles into a continuous film over the contacted area of the substrate.
By way of contrast, in conventional electrostatic spraying processes, the coating powders are charged by blowing them through the nozzle of a spray gun and past the tip of a high-voltage electrode. In a conventional fluidized bed, the fluidizing air is not ionized, and adhesion of the powders is achieved by heating the substrate to the fusion temperature of the coating powders. The substrate is dipped into the bed.
While both ESFB and electrostatic spraying processes are dominated by field charging, there are substantial differences between them. For example, because particles are impelled toward the substrate in electrostatic spraying, there will always be a substantial amount of overspray which, for purposes of economy, must be collected and re-introduced into the system. In the ESFB process, however, because there is no velocity imparted to the particles to carry them beyond the free surface of the fluidized bed, transportation to the substrate is almost exclusively a result of electrostatic forces. Essentially, this means that there is little or no overspray, and non-adhering particles merely fall back into the fluidized bed where they are again charged and immediately made available for redeposition on the substrate.
Because of the pure electrostatic nature of ESFB, the rate of deposition may be faster and the coating thickness may be greater than can be obtained in other electrostatic coating processes. The characteristics of ESFB make it particularly suitable for applying functional coatings of substantial thickness that may be useful, for example, in providing insulation for various electrical devices.